Field of the Invention
The invention relates to seismic instrumentation in general. More particularly, the invention is related to improved fabrication and packaging techniques for a micro-machined suspension plate having an integral proof mass and a method of fabrication for the same that may be utilized in a seismometer (velocimeter), an accelerometer, or other similar device.
Description of the Prior Art
U.S. Pat. No. 6,776,042 discloses a novel construction of an accelerometer or seismometer using an in-plane suspension geometry having a suspension plate and at least one fixed capacitive plate. In contrast to conventional seismometers, which utilize a spring supporting a distinct proof mass on an external frame, the micro-machined suspension plate is formed from a single piece of material to include the external frame, a pair of flexural elements and an integral proof mass interposed between the flexures. The flexural elements allow the proof mass to move in one direction, the sensitive direction, in the plane of suspension, while restricting as far as possible movement in all the other off-axis directions.
The new in-plane design also includes a displacement transducer for determining relative motion of the proof mass. This transducer includes accurately placed drive electrodes, preferably positioned on the proof mass, and corresponding pickup electrodes located on the fixed capacitive plate.
The device further includes either electrostatic or electro-magnetic actuators that can be used within an electronic control loop to re-center the proof mass by exerting a restoring force in a so called force re-balance control system.
U.S. patent application Ser. No. 10/851,029 entitled “IMPROVED MICRO-MACHINED SUSPENSION PLATE WITH INTEGRAL PROOF MASS FOR USE IN A SEISMOMETER OR OTHER DEVICE” discloses improvements to the basic design including adding intermediate frames disbursed within and between the flexural elements in order to produce a system where the frequency of the off-axis modes are as many multiples as possible of the resonant frequency of the system, while minimizing the reduction in the frequencies of spurious modes along the sensitive axis.
The intermediate frames are provided with motion stops, so that under overload conditions the frames engage each other, preventing further relative motion, before the flexures make any contact or become overstressed. These stops thus minimize the chance of fracture or the irreversible surface bonding of portions of the flexure (“stiction”). The structure includes a dampening structure that is highly effective during non-powered/non-operational states (i.e. when the feedback control system is not powered and does not provide any dampening). This dampening structure includes a spring/gas dampening structure configured to provide damping during non-powered states.
For the packaging of such a device, it is well known that melted solder balls can be used to form conducting interconnects off a microelectronic device (chip) and between chips. This knowledge is widely used to bond, and form electrical connections between, a chip and a substrate. The substrate generally provides conductive paths to and from the chip, or chips, bonded in this way to it. This technique has been extended to the packaging of micro-electro-mechanical (MEMS) devices (U.S. Pat. Nos. 6,808,955, 6,852,926, 6,903,452, and 7,030,432) in which as well as producing electrical and mechanical interconnects, the solder balls are disposed between two wetting rings on separate MEMS substrates, so that reflow of the solder balls forms a contiguous ring producing a sealed cavity. A design that uses solder balls to both form a seal and control the spacing of the substrates, has not been presented.
The use of apertures through a frame to position solder balls onto a substrate is known (U.S. Pat. No. 6,857,183). The use of a carrier to hold solder balls within recesses using a vacuum force is also known (U.S. Pat. No. 6,916,731). Controlling the depth of the frame, or recess, to allow only one solder ball to occupy the desired position has been explained in both U.S. Pat. Nos. 6,857,183 and 6,916,731. Neither of these techniques has been extended to the scale of the current invention. Furthermore, there is no prior art for self-aligning the solder balls directly on the substrate to be bonded.
The use of thin micro machined bridges to support MEMS structures while thermally isolating them from their environment is known (U.S. Pat. No. 6,900,702). However, the design for a device that provides maximum thermal isolation utilizing a minimum of substrate area, and maximizing mechanical rigidity, has not been presented.